The invention relates to an optical arrangement, in particular to a microlithographic projection printing installation, in particular having a slot-shaped image field or rotationally non-symmetrical illumination,
a) comprising an optical element;
b) comprising a projection light source which emits radiation, wherein the surface of the optical element is acted upon by the radiation of the projection light source in a rotationally non-symmetrical manner;
c) and comprising a compensating light supply device which supplies compensating light to the optical element in such a way that the temperature distribution in the optical element arising as a result of cumulative heating of the optical element with projection light and compensating light is at least partially homogenized.
The imaging quality of an optical arrangement, which is acted upon in a rotationally non-symmetrical manner by light, is often impaired by rotationally non-symmetrical image defects. Such image defects arise, for example, not only as a result of rotationally non-symmetrical light-induced heating of the, with regard to the projection light, refractive or reflective optical element but also as a result of other light-induced effects, such as e.g. compaction, which lead to a corresponding rotationally non-symmetrical expansion and/or refractive index distribution in the optical element. When high imaging quality is required, as it is in particular for microlithographic projection printing processes, the described light-induced image defects cannot be tolerated.
From the generic EP 0 823 662 A2 an optical arrangement of the type described initially is known, in which by means of the use of a compensating light source an attempt is made to achieve an at least partial reduction of such image defects. This is effected by a homogenization of the temperature distribution in the optical system via the absorption of the compensating light which is effected there. The compensating light is in said case guided parallel to the optical axis through edge regions of the optical elements which are not acted upon by projection light. As a result, the effective aperture of the optical arrangement which is usable for projection printing is restricted. The necessary input coupling of the compensating light parallel to the optical path of the projection light leads additionally to structural integration problems because additional input coupling and/or deflection elements have to be inserted into and/or adjacent to the optical path of the projection light.
The object of the present invention is therefore to develop an optical arrangement of the type described initially in such a way that the temperature distribution in the optical element may be rendered symmetrical and/or homogenized through the use of compensating light without adversely affecting the usable aperture.
Said object is achieved according to the invention in that the compensating light supply device is optically coupled via the peripheral surface of the optical element to the latter.
The input coupling of the compensating light via the peripheral surface leads to the possibility of full utilization of the aperture of the optical arrangement for the projection light because a restriction caused by the compensating light beam guidance is avoided. As the optical paths of projection light and compensating light now no longer extend adjacent or parallel to one another, the optical arrangement may be structurally rectified. In addition, the peripheral surface of the optical elements may be designed independently of the optical surfaces for the projection light so that guidance of the compensating light may be optimized independently of guidance of the projection light. Since optical elements generally have a greater dimension perpendicular to the optical axis than parallel thereto, with input coupling via the peripheral surface there is mostly also a greater material distance available for absorption of the compensating light, with the result that greater freedom exists when selecting the wavelength of the compensating light.
The compensating light supply device may comprise a light source and at least one optical fibre, in which the radiation emitted by the light source is supplied to the optical element. Given the use of a light source, which is independent of the projection light source, it may be accommodated spatially independently of the optical arrangement. With optical fibres it is possible to realize a structural design of the input coupling into the peripheral surface of the optical element which, as a rule, does not lead to a substantial increase of the cross section of the optical arrangement. The output divergence from optical fibres may be utilized to irradiate a relatively large region of the optical elements with compensating light.
Advantageously, at least two optical fibres may be provided and the light outputs guided in each case through said at least two optical fibres may be adjustable independently of one another by means of a control device. By means of such a distribution of the light outputs guided in the individual optical fibres a purposeful influencing of the temperature distribution generated in the optical element by absorption of the compensating light is possible for compensating image defects.
The control device may have a communication link to a sensor monitoring the focal plane of the optical arrangement and may process the signals received from the sensor for control of the light output. In said manner a regulation of the imaging quality is possible, whereby imaging quality changes detected by the sensor are automatically corrected.
The sensor may be a position-sensitive sensor. Such sensors are available in a very inexpensive design, e.g. in the form of quadrant detectors.
The sensor is preferentially a CCD array. Such a sensor guarantees very sensitive determination of the imaging quality of the optical arrangement. A relatively simple construction of the control device is possible, in the present case, through the use of known image processing algorithms.
In a refinement of the invention, the compensating light supply device comprises a light source of variable wavelength. The wavelength provides an additional degree of freedom when adjusting a temperature distribution in the optical element to compensate image defects. For, given the use of a light source having a wavelength, which is adjustable in a range, in which the absorption coefficient of the material of the optical element significantly changes, by changing the wavelength it is possible to realize a change of the depth of penetration of the compensating light into the optical element and hence a corresponding change of the temperature distribution in the latter. Typical wavelength regions, which may be used here, are the long-wave absorption edge in quartz glasses in the region of 4 xcexcm or a regionxe2x80x94occurring in many quartz glassesxe2x80x94of increased intrinsic absorption at 1400 nanometers of a wavelength which may be achieved e.g. by an indium-phosphide diode laser.
In a further refinement of the invention, a holding component for the end of the at least one fibre directed towards the optical element is attached to a mount for the optical element. This leads to a reliable positioning of the output end of the fibre relative to the optical element. Given the use of a detachably mounted holding component, easy exchange of the fibre and easy repositioning of the replacement fibre is guaranteed.
For guidance of the holding component in peripheral direction of the optical element an adjusting device may be provided. Such an adjusting device may be used to adjust both the position, at which the compensating light is coupled into the optical element, and the input coupling direction or alternatively the distance of the output end from the peripheral surface of the optical element. Said degrees of freedom allow an additional influencing of the intensity distribution of the compensating light in the optical element and hence, via the absorption of the compensating light, an influencing of the temperature distribution in said optical element.
For the adjusting device a motor-driven actuator may be provided, which has a communication link to a control device, which in turn has a communication link to a sensor monitoring the focal plane of the optical arrangement and processes the signals received from the sensor for control of the position of the holding components. Thus, automatic correction of image defects is enabled through adjustment of the holding components.
The peripheral surface of the optical element may have facets at compensating light input regions. Such facets enable guidance of the compensating light beam as a result of the refraction at the facet surface. The facets may be e.g. convex, leading to a concentration of compensating light impinging divergently on the facets. If, on the other hand, the shape of the facets is concave, a divergence of the impinging compensating light beam may be realized. When the radius of curvature of concave facets corresponds to the divergence of impinging compensating light such that the compensating light rays meet the facet surface at right angles, there is no influencing of the divergence by refraction at the peripheral surface of the optical element.
The peripheral surface of the optical element may be textured at compensating light input regions. In the simplest case, such texturing is provided by the normally roughly ground peripheral surface of the optical element. The compensating light striking a textured peripheral surface is diffused, thereby promoting the distribution of the compensating light in the optical element. Other types of texture of the peripheral surfaces for influencing the guidance of the compensating light beam are conceivable, e.g. in the manner of a diffractive optical element.
When the emission wavelength of the light source is greater than 4 xcexcm, a relatively high absorption of the compensating light is guaranteed. In said case, in order to achieve a specific heat output, for the compensating light it is therefore possible to use light sources with a relatively low optical output.
The optical element may be a refractive optical element. Such refractive optical elements, e.g. in the form of lenses or plane-parallel plates, are standard equipment in known projection printing installations.
Alternatively, the optical element may be reflective as regards the radiation of the projection light source. Because of residual absorptions of the projection light in the reflective surface such a mirror for the projection light also experiences a heat contribution, which substantially presents the symmetry of the impingement with projection light. When the mirror in accordance with the invention is designed in such a way that the compensating light supply device is coupled to its peripheral surface, then, here too, an image defect induced by the projection light may be compensated by absorption of the compensating light. A typical realization of such a mirror comprises a reflective coat on a transparent substrate.